Polytrimethylene terephthalate (PTT) is prepared by the polycondensation reaction of 1,3-propanediol with terephthalic acid or terephthalic acid esters. PTT polymer and copolymers thereof are finding ever increasing commercial use in the preparation of fibers, fabrics, carpets, films, molded parts and the like.
Standard practice in the polymer art is to manufacture thermoplastic polyesters in a “continuous polymerizer” extruding the produced molten polymer though an extruder equipped with a so-called “strand die” which is simply a metal plate having one or more about ⅛-¼″ circular cross-section holes in it. A continuous strand, about 1/10-⅛″ in diameter, of molten thermoplastic polymer is extruded out of each hole of the strand die. Immediately upon discharge from the strand die, the hot strand is usually passed through a water quench region normally consisting of a water quench tank or a water spray chamber. The length of the quench region is largely dependent upon the linear rate of extrusion of the strand—that is, how fast the strand is moving—and the melt temperature of the strand at the exit of the die. At the end of the quench region, the strand is separated from the water, and is directed to a cutter that cuts the strand into pellets about ⅛″ long.
This practice, developed for poly(ethylene terephthalate) (PET), has been applied to poly(trimethylene terephthalate) (PTT). Because of the rapid quenching of the strand, the resulting pellets tend to exhibit amorphous (that is, non-crystalline) surfaces. This is not a problem in PET because the glass transition temperature is sufficiently high that the amorphous surfaces remain solid until the pellet is further melt processed. PTT exhibits a glass transition temperature (Tg) of about 45° C.—much lower than that of PET and other well-known polyesters such as PBT or PEN. It has been found that when PTT pellets prepared by the process described supra are stored where the ambient temperature reaches about 40° C., some degree of surface polymer flow may occur on a microscopic scale, thereby causing stored pellets to stick together, forming aggregates that hinder the smooth flow of pellets during processing. By inducing surface crystallization, pellets can be produced that exhibit greatly reduced sticking and aggregation when subject to relatively high storage temperatures.
Hanniman et al., US2009/0057935, discloses a process for producing a non-adhering granule from a polyester above Tg by introducing a granulated material into cooling water at 80-110° C., where the granulated material is prepared from a melt and fed to an underwater melt cutter.
Duh et al., U.S. Pat. No. 6,297,315, discloses a process for crystallizing PTT before pelletizing by immersing the quenched strand into a water batch heated to 60-100° C. Duh teaches that best results are obtained by crystallization after pelletization. Further, according to Duh, a water temperature of 60° C. requires a 20 minute crystallization time to achieve satisfactory results. This is because, as Duh shows, 60° C. is below the crystallization temperature of about 69° C.
Nishiyama et al, JP7223221 (A) (abstract only), discloses a process suitable for use with a polyester having a Tg less than 40° C. in which a molten strand is quenched, then subject to stretching to induce crystallization therein, followed by pelletization.
Shelby et al., U.S. Pat. No. 6,159,406, discloses a process for crystallizing difficult to crystallize polyester melts before pelletizing by quenching the strand, and then stretching the strand by winding on two sets of godets separated by a hot water bath set above Tg through which the strand passes.